Method for producing highly ductile metallic coated ferrous sheet and strip



United States Patent 3,228,810 METHOD FOR PRODUCING HIGHLY DUCTILE METALLIC COATED FERROUS SHEET AND STRIP Kenneth G. Coburn, Middletown, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio, 21 corporation of Ohio No Drawing. Filed Mar. 1, 1963, Ser. No. 262,202 7 Claims. (Cl. 148-16) My invention relates to the manufacture of a ferrous metal product having a thin surface coating of zinc or zinc alloys, aluminum or aluminum alloys, terne, and the like, and more particularly to a process which will provide a finished product having greatly enhanced drawability.

Processes for continuously coating a ferrous base metal, such as the well known Sendzim-ir process taught in United States Patent No. 2,110,893, will produce a coated prodnot capable of withstanding normal drawing operations. Even when practiced on full hard cold reduced iron or steel strip, this process causes the base metal to recrystallize into a product having substantial softness and ductility.

More recently, however, various industries have begun deep drawing and forming articles having compound curvatures and the like; and this has resulted in an acute need for a still softer and more ductile coated product.

It has heretofore been impossible to achieve this greater drawability under conventional practice of the Sendzimir coating process. This is because the temperature required to prep-are the surface of the full hard cold reduced base metal and to effect good adhesion with the coating metal is so high that the resulting product will exhibit normalized properties. The strip temperature will exceed the A critical temperature in many cases.

In other words, the Sendzimir process contemplates the formation of a thin, controlled layer of oxide on the surface of a steel strip, reducing that oxide film and then, while protecting the strip from the external atmosphere, dipping it into a bath of coating metal. One of the most important aspects of this treatment is the thorough cleaning of the surface of the base metal to be coated to remove the contaminants, including oil and grease which it had acquired in the cold rolling process so that the coating metal will adhere to the base metal. Commercial practice has clearly established that certain minimum temperatures are necessary to satisfactorily accomplish this cleaning operation.

On the other hand, it has also been recognized that drawing quality box annealed steel strip containing .03- ;10 percent carbon which has excellent drawing characteristics, will lose some of its desirable mechanical properties when heated above a certain definable temperature and air cooled. That is to say, there is a maximum temperature to which such box annealed strip can be sub jected without reducing its drawability. Furthermore, normal operation of the continuous anneal-coating sequence of operations utilized in the Sendzimir process involves temperatures above this maximum which will reduce the drawability of the base metal and often result in a product having substantially normalized properties. In fact, laboratory tests have indicated that even if such box annealed stock is run through a furnace in which the strip temperatures are held as low as about 1350 F., there will be a significant loss of the desired mechanical properties.

It is the problem of satisfactorily cleaning the steel strip to be coated without loss of desirable drawing characteristics which has been solved in a commercially feasible process by the instant invention. Heretofore, these two temperature requirements could not be met in the same process because the minimum temperature necessary for cleaning is above the maximum temperature which 3,228,810 Patented Jan. 11, 1966 ICC will permit retention of deep drawing properties. Hydrogen reduction must be carried out at 1350 to 1800 F. and, as pointed out above, such temperatures will result in a reduction in drawing quality of box annealed low carbon steels.

Accordingly, it is the principal object of this invention to provide a procedure which is commercially practicable, and which will produce a coated iron or steel strip having greatly enhanced drawability.

Another very important object of this invention is the provision of a process for increasing the drawability of a metallic coated cold rolled steel strip which may be used in conjunction with a continuous coating process.

These fundamental objects, along with others which will become apparent to the skilled worker in the art as this specification proceeds, are accomplished by a certain series of process steps which will now be described.

Briefly, in the preferred practice of this invention, the base metal will be treated by the opened coil annealing process to reduce the carbon content to 015% or less. After a light temper rolling, the stock will be coated in a continuous coating line in which the strip passes through a reducing furnace. The strip temperature is kept within certain precisely defined limits set forth in greater detail hereinafter. By way of example, a product made according to the instant process may have the following mechanical properties-the yield strength will be about 25,000 p.s.i., elongation about 50%, R hardness between 35 and 45 and a grain size of ASTM number 6 to 9. This may be compared with normalized stock which will have a yield strength of about 40,000 p.s.i., elongation of about 30% and R hardness of about 60, along with somewhat smaller grain size.

In practicing the present invention, the starting material may be either rimmed, capped or killed steel preferably in the form of a continuous cold reduced strip. The phrase cold reduced strip simply refers to a ferrous product which has been reduced to final sheet gauge by a substantial cold reduction, as distinguished from a mere temper rolling performed on hot rolled steel sheets.

The first step in the instant process involves decarburizing the steel strip by means of the opened coil annealing process. As will be understood by the skilled worker in the art, the opened coil annealing process involves winding or coiling the steel strip with a strand of wire or other spacing material between the coil convolutions. This wire strand or other material may then be removed to leave a predetermined amount of air space between each coil convolution, or it may be left in place during annealing. The strip coiled in the above manner is then heated to approximately 1350 F. in a controlled atmosphere, preferably a relatively dry hydrogen bearing atmosphere, as for example dissociated ammonia. When using such an atmosphere, decarburization is accomplished by introducing steam once the coiled strip reaches the desired temperature. However, it will of course be understood that the decarburizing atmosphere may be generated in any conventional manner. The decarburization may be carried out at temperatures between 1100 F. and the A critical temperature. For practical purposes, the preferred operating range will be between 1300" F. and 1500 F.

The decarburization should continue until the carbon content of the steel strip is at most 015%. It has been discovered that this degree of decarburization accomplishes two very important results. (1) Steel strip decarburized in this manner may be heated to a higher maximum temperature (a temperature which would normalize ordinary stock) without losing its drawability and mechanical properties, and (2) its surfaces are more easily cleaned, and hence the coating line may be operated under lower temperature conditions. The importance of these aspects of this discovery in view of the preambular discussion should be immediately apparent.

After cooling the decarburized coiled strip to substantially room temperature, it has been found desirable to subject the strip to a temper rolling operation. This step will of course, introduce some degree of strain into the strip and increase its hardness slightly, but greatly increases its handling qualities, and so it most desirable from that standpoint.

Cleaning the surface of the strip preparatory to the coating operation may be accomplished in several ways. As taught in the above noted Sendzimir patent, the steel strip is passed through a short oxidizing furnace wherein sufficient heat is supplied to burn the oil and grease from the surface of the steel strip. Commercial practice has established that a range of strip temperatures from 700 to 900 F. is quite satisfactory,

It is possible to omit this oxidizing step under some circumstances where the surface is unusually clean. For example, a strip which has been bright annealed and is fed directly to the metallic coating unit could be prepared for coating without using the oxidizing step described above. In the alternative, the strip may be chemically cleaned by any of the well known conventional practices such as acid pickling or alkaline cleaning.

The strip must next be passed through a reducing furnace. While several types of furnaces could be employed for this purpose, preferred practice of this process requires the use of a continuous strand type reducing furnace which, according to conventional practice, includes a heating portion and a controlled cooling portion. It will also be apparent that, if desired, the oxidizing furnace and the reducing furnace could be combined into a sinble structure simply by providing an oxidizing atmosphere at or near the entrance end, and a reducing atmosphere at or near the exit end, and separting these atmospheres by baffle plates or the like so as to minimize gas diffusion.

The reducing furnace will reduce or remove any thin oxide coating and reconvert it into a layer of the base metal of the strip which is pure in the sense of being free from carbon, oxides, and the like. This reduced layer tightly adheres to the surface of the base metal, and makes the base metal particularly receptive to the coating metal.

The reducing atmosphere may be generated in any conventional fashion, and the necessary quantities of reactants determined from Well known equilibrium data.

In conventional Sendzimir practice, and depending on certain conditions, the maximum strip temperature in the reducing furnace may be varied from 1350 to 1800 F. However, commercial practice has determined that temperatures near the upper end of this range are definitely preferable, primarily because of the superior cleaning capabilities inherent in the higher temperatures.

In the instant invention, the temperature of the strip in the reducing furnace is of critical importance, and must be carefully controlled. It has been discovered that by maintaining the strip temperature below the A critical point for the decarburized steel strip being proc essed, the mechanical properties and grain texture imparted to that steel by the opened coil annealing step may to a large extent be retained. The exact temperature limitation will of course vary with the carbon content of the steel being processed, but in any event, the temperature must not exceed the A critical point for the steel in question. This upper temperature limitation may be more precisely defined as the temperature at which grain growth will occur to produce large grain size which greatly impairs drawability. 'Usually, the predominant grain size for a drawable steel will have to be smaller than ASTM Number 6. Preferred practice utilizes temperatures near the upper limit, although lower temperatures will give satisfactory results on suitable starting material. It has been empirically determined that for general commercial operations, the practical maximum temperature is about 1575 F. The lower temperature limit is determined by the requirement for surface preparation and for the normal Sendzimir process, would be about 1350 F.

Finally, Without reexposing the steel strip to the air, it is passed beneath the surface of a molten bath of coating metal. A simple cooling snout of conventional construction extending from the exit portion of the reducing furnace to the bath of coating metal will suffice to prevent reexposure to the external atmosphere while the strip is being cooled to a temperature approximating that of the coating metal. While it is possible (in theory at least) to have the atmosphere in the hood of a neutral character, a reducing atmosphere will preferably be supplied.

It is also to be preferred that the hood and furnace be maintained at a pressure somewhat in excess of atmospheric pressure. Since the hood will be sealed at one end by the bath of coating metal, the excess pressure will cause some of the reducing gas to burn out at the entrance end of the reducing furnace, thereby preventing contamination of the furnace atmosphere by the external atmosphere.

In order to illustrate a specific application of the process of this invention, the following example is given. The pertinent chemistry of the strip being processed included:

Percent Carbon .06

Manganese .33 Phosphorous .004 Sulphur .024 Silicon .003

Copper .038 Aluminum .040

Iron Balance Decarburization was accomplished by an opened coil anneal for nine hours at 1300 F. The atmosphere consisted of 40% H and 60% N at a dew point of F. After decarburization, chemical analysis showed that the carbon content of the strip had been reduced to .0018%, while the amounts of the other components remained essentially the same.

Before entering the coating line, the strip was subjected to a 1% temper roller to improve its handling qualities.

In a Sendzimir line, the strip temperature in the oxidizing furnace was maintained at 800 F., and the strip temperature in the reducing furnace was held at 1550 F. In this particular run, the coating metal was zinc, but this should not be construed as a limitation. Since this invention discloses a process for treating the base metal, the decarburizing, oxidizing and reducing steps will be the same for all coating metals.

After coating, the strip was subjected to a .5 temper roll, and the following mechanical properties were obtained:

Yield strength p.s.i. 27,800 R hardness 42 Elongation in 2" percent 44 ASTM grain size 8-9 If desired, the coated strip can be subjected to further process steps for the purpose of treating the coated surface. Such subsequent steps are entirely conventional and are well known to the skilled worker in the art.

It should now be apparent that numerous modifications can be made in the instant invention without departing from its scope or spirit. Accordingly, no limitation is intended except as set forth in the following claims.

Having thus described this invention, what is claimed as new and what is desired to be secured by Letters Patent is:

1. A process for producing a metallic coated ferrous metal strip having greatly enhanced drawability comprising the steps of forming said strip into an opened coil, raising the temperature of said coiled strip in a controlled atmosphere, and decarburizing said heated opened coiled strip to a carbon content of less than .015 subjecting said strip to heat to raise the temperature of said strip to the range of 700-900 F. and to clean the surface thereof, and thereafter further heating said strip to a temperature in the range between 1350* F. and the A critical point of the strip in a reducing atmosphere, and passing said strip into a bath of molten coating metal without re-exposing said strip to the air.

2. The process claimed in claim 1 including the step of temper rolling and decarburized strip prior to its oxidizing and reducing heat treatment.

3, The process claimed in claim 2 wherein the steps of subjecting the strip to heat under oxidizing conditions and of thereafter heating said strip to a temperature in the range between 1350 F. and the A critical point of the strip in a reducing atmosphere are accomplished in continuous strand type annealing furnaces.

4. The process claimed in claim 3 wherein the decarburization is accomplished in an atmosphere containing hydrogen and water vapor.

5. A process for producing a metallic coated ferrous metal strip having greatly enhanced drawability comprising the steps of cold rolling said strip to a finished gauge by a substantial cold reduction, forming said strip into an opened coil, raising the temperature of said opened coiled strip in a relatively dry, hydrogen bearing atmosphere, introducing water vapor into said hydrogen bearing atmosphere to effect decarburization of said coiled strip, and continuing said decarburization until the carbon content of said coiled strip is at most .015 cooling said decarburized coiled strip, thereafter subjecting said strip to heat under oxidizing conditions to raise the temperature of said strip to the range of 700900 F. and to clean the surface thereof, then heating said strip to a temperature above 1350 F. and below 1575 F. in a reducing atmosphere, and then passing said strip into a bath of molten coating metal without re-exposing said strip to the air.

6. The process claimed in claim 5 wherein the steps of subjecting the strip to heat under oxidizing conditions and of heating said strip to a temperature above 1350 F. and below 1575 F. in a reducing atmosphere are accomplished in continuous strand type annealing furnaces.

7. A process for producing a metallic coated ferrous base metal having greatly enhanced drawability comprising the steps of forming said base metal into an open coil and decarburizing said base metal to a carbon content of less than .015 thereafter cleaning the surface of said base metal in preparation for coating while maintaining said base metal at a temperature in the range of 1350 F. and the A critical point of the base metal, and then passing said base metal into a bath of molten coating metal.

References Cited by the Examiner UNITED STATES PATENTS 2,110,893 3/1938 Sendzimir 11771 2,360,868 10/1944 Gensamer 148-16 3,105,780 10/1963 Low 14816 HYLAND BIZOT, Primary Examiner.

DAVID L. RECK. Examiner. 

1. A PROCESS FOR PRODUCING A METALLIC COATED FERROUS METAL STRIP HAVING GREATLY ENHANCED DRAWABILITY COMPRISING THE STEPS OF FORMING SAID STRIP INTO AN OPENED COIL, RAISING THE TEMPERATURE OF SAID COILED STRIP IN A CONTROLLED ATMOSPHERE, AND DECARBURIZING SAID HEATED OPENED COILED STRIP TO A CARBON CONTENT OF LESS THAN .015%, SUBJECTING SAID STRIP TO HEAT TO RAISE THE TEMPERATURE OF SAID STRIP TO THE RANGE OF 700*-900*F. AND TO CLEAN THE SURFACE THEREOF, AND THEREAFTER FURTHER HEATING SAID STRIP TO A TEMPERATURE IN THE RANGE BETWEEN 1350*F. AND THE A3 CRITICAL POINT OF THE STRIP IN A REDUCING ATMOSPHERE, AND PASSING SAID STRIP INTO A BATH OF MOLTEN COATING METAL WITHOUT RE-EXPOSING SAID STRIP TO THE AIR. 